Many geological, biological and other solid materials are heterogeneous composite structures formed from interrelated, but microscopically and chemically discrete particles or cells. The commercial use of these materials can require information related to the chemical and physical properties of the material's individual microscopic particles. However, conventional laboratory analysis of a sample of these composite materials, such as a wellbore core sample is performed on a bulk basis. Analysis of these samples is commonly accomplished by a variety of methods, including fluorescent spectroscopy, fluorometry and fluorescent microscopy.
Fluorescent techniques use an energy source, such as incident continuous wave ultraviolet (UV) irradiation, to excite a sample to cause fluorescent emissions from the sample. Fluorescence is the emission of radiant energy (such as light) as the excited electrons of an atom or molecule within the sample return to a lower or ground state after being promoted to a higher energy state by absorption of the exciting energy. Fluorescent radiations are normally distinct from light absorption, transmission and reflection with respect to time (from absorption of incident irradiation), direction, intensity and wavelength.
Molecules contained in the materials can possess ground state and many excited electron states. Electron transitions between the many electron states cause fluorescent emissions to be at several different intensities, wavelengths, and times after absorption, the emissions forming a spectral structure. The fluorescent spectral structure can be quite complex for materials having heterogeneous chemical structures.
Conventional fluorescence spectroscopy and photometry are commonly used to detect the composition and concentration of organic compounds in diluted solutions. The emission spectra of these compounds, which normally possess de-localized electrons, can display vibrational bands as a result of transitions between the different electron states. However, for a majority of solid composite samples, these techniques may not yield molecularly resolved spectral or intensity information.
If a fluorescent analysis of a composite sample having many constituents is desired, the analysis method and device becomes much more complex. Conventional fluorescence analysis devices typically use continuous wave mercury emission as illumination and excitation source. These are primarily designed for homogeneous samples (typically dissolved in a diluted solution) and are not normally used for composite solid samples without process and/or apparatus changes. These devices bulk illuminate (i.e., energize many particles within the entire sample) and measure bulk fluorescence (i.e., the intensity and spectral emissions of the entire sample), producing potentially overlapping spectral structures from a composite sample.
One composite sample analysis technique involves splitting the sample. A small sample portion is prepared and isolated for optical analysis (microscopic examination) and the other portion of the sample is then used for a separate bulk chemical analysis. This two step process tends to be slow, complex, and unreliable.
Another composite sample analysis technique is fluorescence microscopy. In one embodiment of the fluorescence microscopy method, bulk illumination of several particles of the sample is accomplished, typically by a high pressure mercury arc lamp. The integration of a microscope, a scanning monochromator, and a photomultiplier detector, forms a microscopic detector which can be directed to a relatively small area of interest. With the use of a measuring diaphragm, the focused detection of an illuminated microscopic particle within the sample allows microspectrophotometry to be performed on the fluorescent emissions. Quantitative measurement of the detected fluorescence intensity and spectral distribution provides information regarding one or more fluorescing particles.
However, because several particles of the sample are illuminated, emissions from unwanted particles or portions of the system cannot always be totally excluded. Other sample portions can produce significant fluorescence within the sample which may be emitted towards the focused detector. It may not be possible to accurately segregate the contribution(s) of each type of particle from the mixed detection information generated by this method. The detector focus area of interest may also not be able to be reduced to isolate a single particle's emissions.
Secondly, as a consequence of the broad excitation and absorption bands, continuous-wave excitation, and long exposure time (generally in the order of several milliseconds to several seconds), the fluorescence spectrum obtained by conventional fluorescence microscopy may not be completely resolved, and the spectrum typically provides little chemical information regarding the microscopic particles of interest.
As an alternative to this fluorescence microspectrophotometry method, a continuous wave laser beam can be used as a source of irradiation energy. This is illustrated in U. S. Pat. Nos. 4,616,133. The laser irradiation process essentially irradiates an area of interest within a sample with ultraviolet (UV) radiation from a helium-cadmium or nitrogen laser, separates the resulting emission spectrum into wavelength segments, and measures the intensities within each wavelength segment.
The laser selectively excites certain constituents or particles in the sample. The monochromaticity of the laser also tends to limit the excitation states of specific molecules whose absorption bands coincides with the laser emission. A Xenon lamp coupled with a scanning monochromator provides a tunable continuous wave source which can also be used to selectively excite constituents or selectively achieve certain excitation states. Measured emissions (intensity in a given direction within a wavelength segment) are compared to one or more reference emissions to identify properties of particles in the area of interest.
Prior work indicates that the fluorescent emissions are not always constant, but may change over extended exposure time. Although most fluorescence is relatively rapid, delayed fluorescence or phosphorescence is also common. Exciting irradiation can thus produce overlapping rapid and delayed emissions over time. In addition, other changes in emission intensity and wavelength appear to be caused by absorption of laser energy which is not emitted as fluorescence. Temperature increases, oxidation or other reactions are some of the results of the absorption of laser energy. These time dependent factors can cause significant changes to the spectral structure over time, such that unique spectral time changes can also be used for identification purposes.
Even with microspectrophotometry and the use of a continuous wave laser, the composite samples create spectral analysis problems. First, the use of a continuous wave laser cannot separate fluorescence from slower emissions. The temporal behavior of fluorescence cannot be precisely examined, especially in the first hundred nanoseconds. Second, the continuous excitation causes photochemical reactions of the composite samples which may give rise to chemical information unrelated to the original chemistries. Third, a continuous-wave UV laser produces fluorescence emission with little chemical information, since the fluorescence spectra are almost as broad and unstructured as induced by mercury excitation.